compactibleFreeListSpace.cpp

// Support for concurrent collection policy decisions.
bool CompactibleFreeListSpace::should_concurrent_collect() const {
  // In the future we might want to add in frgamentation stats --
  // including erosion of the "mountain" into this decision as well.
  return !adaptive_freelists() && linearAllocationWouldFail();
}

bool CompactibleFreeListSpace::linearAllocationWouldFail() const {
  return _smallLinearAllocBlock._word_size == 0;
}

HeapWord*
CompactibleFreeListSpace::getChunkFromSmallLinearAllocBlock(size_t size) {
  assert_locked();
  assert(size >= MinChunkSize, "minimum chunk size");
  assert(size <  _smallLinearAllocBlock._allocation_size_limit,
    "maximum from smallLinearAllocBlock");
  return getChunkFromLinearAllocBlock(&_smallLinearAllocBlock, size);
}

HeapWord*
CompactibleFreeListSpace::getChunkFromLinearAllocBlock(LinearAllocBlock *blk,
                                                       size_t size) {
  assert_locked();
  assert(size >= MinChunkSize, "too small");
  HeapWord* res = NULL;
  // Try to do linear allocation from blk, making sure that
  if (blk->_word_size == 0) {
    // We have probably been unable to fill this either in the prologue or
    // when it was exhausted at the last linear allocation. Bail out until
    // next time.
    assert(blk->_ptr == NULL, "consistency check");
    return NULL;
  }
  assert(blk->_word_size != 0 && blk->_ptr != NULL, "consistency check");
  res = getChunkFromLinearAllocBlockRemainder(blk, size);
  if (res != NULL) return res;

  // about to exhaust this linear allocation block
  if (blk->_word_size == size) { // exactly satisfied
    res = blk->_ptr;
    _bt.allocated(res, blk->_word_size);
  } else if (size + MinChunkSize <= blk->_refillSize) {
    size_t sz = blk->_word_size;
    // Update _unallocated_block if the size is such that chunk would be
    // returned to the indexed free list.  All other chunks in the indexed
    // free lists are allocated from the dictionary so that _unallocated_block
    // has already been adjusted for them.  Do it here so that the cost
    // for all chunks added back to the indexed free lists.
    if (sz < SmallForDictionary) {
      _bt.allocated(blk->_ptr, sz);
    }
    // Return the chunk that isn't big enough, and then refill below.
    addChunkToFreeLists(blk->_ptr, sz);
    split_birth(sz);
    // Don't keep statistics on adding back chunk from a LinAB.
  } else {
    // A refilled block would not satisfy the request.
    return NULL;
  }

  blk->_ptr = NULL; blk->_word_size = 0;
  refillLinearAllocBlock(blk);
  assert(blk->_ptr == NULL || blk->_word_size >= size + MinChunkSize,
         "block was replenished");
  if (res != NULL) {
    split_birth(size);
    repairLinearAllocBlock(blk);
  } else if (blk->_ptr != NULL) {
    res = blk->_ptr;
    size_t blk_size = blk->_word_size;
    blk->_word_size -= size;
    blk->_ptr  += size;
    split_birth(size);
    repairLinearAllocBlock(blk);
    // Update BOT last so that other (parallel) GC threads see a consistent
    // view of the BOT and free blocks.
    // Above must occur before BOT is updated below.
    OrderAccess::storestore();
    _bt.split_block(res, blk_size, size);  // adjust block offset table
  }
  return res;
}

HeapWord*  CompactibleFreeListSpace::getChunkFromLinearAllocBlockRemainder(
                                        LinearAllocBlock* blk,
                                        size_t size) {
  assert_locked();
  assert(size >= MinChunkSize, "too small");

  HeapWord* res = NULL;
  // This is the common case.  Keep it simple.
  if (blk->_word_size >= size + MinChunkSize) {
    assert(blk->_ptr != NULL, "consistency check");
    res = blk->_ptr;
    // Note that the BOT is up-to-date for the linAB before allocation.  It
    // indicates the start of the linAB.  The split_block() updates the
    // BOT for the linAB after the allocation (indicates the start of the
    // next chunk to be allocated).
    size_t blk_size = blk->_word_size;
    blk->_word_size -= size;
    blk->_ptr  += size;
    split_birth(size);
    repairLinearAllocBlock(blk);
    // Update BOT last so that other (parallel) GC threads see a consistent
    // view of the BOT and free blocks.
    // Above must occur before BOT is updated below.
    OrderAccess::storestore();
    _bt.split_block(res, blk_size, size);  // adjust block offset table
    _bt.allocated(res, size);
  }
  return res;
}

concurrentMarkSweepGeneration.cpp

// We should be conservative in starting a collection cycle.  To
// start too eagerly runs the risk of collecting too often in the
// extreme.  To collect too rarely falls back on full collections,
// which works, even if not optimum in terms of concurrent work.
// As a work around for too eagerly collecting, use the flag
// UseCMSInitiatingOccupancyOnly.  This also has the advantage of
// giving the user an easily understandable way of controlling the
// collections.
// We want to start a new collection cycle if any of the following
// conditions hold:
// . our current occupancy exceeds the configured initiating occupancy
//   for this generation, or
// . we recently needed to expand this space and have not, since that
//   expansion, done a collection of this generation, or
// . the underlying space believes that it may be a good idea to initiate
//   a concurrent collection (this may be based on criteria such as the
//   following: the space uses linear allocation and linear allocation is
//   going to fail, or there is believed to be excessive fragmentation in
//   the generation, etc... or ...
// [.(currently done by CMSCollector::shouldConcurrentCollect() only for
//   the case of the old generation; see CR 6543076):
//   we may be approaching a point at which allocation requests may fail because
//   we will be out of sufficient free space given allocation rate estimates.]
bool ConcurrentMarkSweepGeneration::should_concurrent_collect() const {

  assert_lock_strong(freelistLock());
  if (occupancy() > initiating_occupancy()) {
    if (PrintGCDetails && Verbose) {
      gclog_or_tty->print(" %s: collect because of occupancy %f / %f  ",
        short_name(), occupancy(), initiating_occupancy());
    }
    return true;
  }
  if (UseCMSInitiatingOccupancyOnly) {
    return false;
  }
  if (expansion_cause() == CMSExpansionCause::_satisfy_allocation) {
    if (PrintGCDetails && Verbose) {
      gclog_or_tty->print(" %s: collect because expanded for allocation ",
        short_name());
    }
    return true;
  }
  if (_cmsSpace->should_concurrent_collect()) {
    if (PrintGCDetails && Verbose) {
      gclog_or_tty->print(" %s: collect because cmsSpace says so ",
        short_name());
    }
    return true;
  }
  return false;
}